In an environment in which a speed sensor cannot be attached or in order to reduce the cost of a control system, conventionally, a speed sensorless control has variously been used. At present, a V/F control is employed in uses in which a speed responsiveness is not required, while a sensorless vector control is employed in uses in which the high speed responsiveness is required. The sensorless vector control serves to estimate a speed without using the position and speed sensors and to implement torque and speed controls in the same manner as a control having a sensor. A method for estimating the speed of a synchronous motor which has mainly been employed utilizes an induction voltage generated corresponding to a rotation (an induction voltage method). There has been a method using a state observer in addition to the induction voltage method “Lang, U.S. Pat. No. 5,296,793, issued Mar. 22, 1994”. In the method, a phase voltage and a phase current in a synchronous motor are detected and are input to a state observer constituted by an expression model based on the electromechanical equation of the motor to estimate the phase angle of the rotor of the synchronous motor. Moreover, a sensorless control based on the voltage and current equation of a brushless DC motor has been reported in “Matsui et al., “Sensorless operation of brushless DC motor drives,” Proc. IEEE International Conference on Industrial Electronics, Control, and Instrumentation, vol. 2, pp. 739–744, 1993”. However, these methods are constituted by an electric motor model. Therefore, there is a problem in that a sensitivity to a fluctuation in an electric motor parameter is high so that a deterioration in a control is caused by a voltage error. For this reason, particularly, it becomes impossible to estimate a speed in a very low speed region, including a zero speed, under a low induction voltage.
There is a method utilizing the magnetic saliency of an electric motor when estimating the speed at a very low speed including the zero speed. A voltage or current signal having a frequency component other than the output frequency of the electric motor is applied to the electric motor, thereby observing a change in an inductance or an impedance of the stator winding through the magnetic saliency to detect a position and a speed.
As one method, there has been disclosed “Ogasawara et al., “Implementation and position control performance of a position sensorless IPM motor drive system based on magnetic saliency,” IEEE Trans. Ind. Appl., vol. 34, pp. 806–812, July/August 1998.”. In this method, an inductance is detected in a short sampling time by using a voltage signal. For this reason, there is a problem in that the method is apt to be influenced by a fluctuation in a parameter or an observed noise. As another method, there has been disclosed “Jansen et al., “Transducerless position and velocity estimation in induction and salient AC machines,” IEEE Trans. Ind. Appl., vol. 31, pp. 240–247, March/April 1995.” for observing the position of a magnetic flux by using a high frequency signal rotating synchronously with speed of the rotations of the electric motor. However, a dynamic characteristic is limited due to the high frequency signal rotating synchronously with speed of the rotations of the electric motor being used. As a method for superposing a high frequency on an induction motor to estimate a speed, furthermore, there has been disclosed “Ha et al., Sensorless field-orientation control of an induction machine by high-frequency signal injection,” IEEE Trans. Ind. Appl., vol. 35, pp. 45–51, January/February 1999.”. This method can also be applied to a synchronous motor. Examples of an application to a reluctance motor include “Ha et al., Position-controlled synchronous reluctance motor without any rotational,” IEEE Trans. Ind. Appl., vol. 35, pp. 1393–1398, November/December 1999.” and examples of an application to a permanent magnet internal embedded type electric motor include “Ha et al., Sensorless position control and initial position estimation of an interior permanent magnet,” Proc. IEEE Industry. Applications Conference, 2001.”.
This method serves to superpose a high frequency, thereby estimating the position of a magnetic flux from a primary voltage and a current in an electric motor. In this method, a signal to be superposed is superposed on a magnetic flux axis rotating synchronously with an output frequency and is different from a high frequency signal rotating synchronously with speed of the rotations of the electric motor described above, and a superposed frequency is independent of an inverter output frequency. It becomes possible to estimate the position of the magnetic flux by extracting a high frequency impedance generating a magnetic saliency. Since the high frequency is superposed on the magnetic flux axis, a torque ripple is comparatively lessened so that a noise is also reduced. According to the method, it is possible to implement torque, speed and position controls at a zero speed and a zero output frequency.
Moreover, examples of the estimation of the speed of an induction motor and a vector control method include “Schauder, U.S. Pat. No. 4,862,054, issued Aug. 29, 1989”. In the example, two standard models based on the electric circuit equation of an electric motor are mounted. One of them is a voltage model based on the primary side equation of the electric motor and the other is a current model based on a secondary side equation. Each of the models is constituted in a rectangular coordinate system. The speed is adaptively estimated by using a PI controller in such a manner that a deviation between two magnetic fluxes calculated from the models is zero. Moreover, a method for adaptively estimating a speed by using a magnetic flux observer has been reported in “Kubota et al., “Speed Sensorless Field Oriented Control of Induction Motor with Rotor Resistance Adaptation,” IEEE Trans. on Ind. Appl., Vol. 30, No. 5, pp. 1219–1224, September/October 1994,”. In this document, there has been proposed a method for adaptively estimating a speed in order to set the vector product value of an output error between an observer and a system, corresponding to the torque error of the observer, and a magnetic flux estimation value to be zero, which is performed simultaneously with the estimation of a magnetic flux.
However, these methods are constituted by an electric motor model. For this reason, there is a problem in that a sensitivity to a fluctuation in an electric motor parameter is high so that a deterioration in a control is caused by a voltage error. In particular, they are apt to be unstable in a region having a zero speed and a zero output frequency. An investigation into the problems of the stability of a sensorless control has been reported in ‘Harnefors, “Instability Phenomena and Remedies in Sensorless Indirect Field Oriented Control,” IEEE Trans. on Power Elec., Vol. 15, No. 4, pp. 733–743, July, 2000’ and ‘Sugimoto et al., “A Consideration about Stability of Vector Controlled Induction Motor Systems Using Adaptive Secondary Flux Observer,” Trans. of IEE Japan, Vol. 119–10, No. 10, pp. 1212–1222, 1999.’.
As one method for solving the problems of the sensorless control in the region having a zero speed and a zero output frequency, there has been proposed ‘Sul et al., “Sensorless Field Orientation Control Method of an Induction Machine by High Frequency Signal Injection,” U.S. Pat. No. 5,886,498, issued Mar. 23, 1999.’. In the method, a high frequency is superposed to estimate the position of a secondary magnetic flux from a primary voltage and a current in an electric motor in the same manner as in case of the synchronous motor. A signal to be superposed is superposed on a magnetic flux axis rotating synchronously with an output frequency and a superposed frequency is independent of the output frequency. It is possible to estimate the position of the magnetic flux by extracting a high frequency impedance generating a saliency magnetically. The principle of the magnetic saliency has been investigated in ‘Ha et al., “Physical understanding of high frequency injection method to sensorless drives of an induction machine,” Proc. IEEE Industry Applications Conference, Vol. 3, pp. 1802–1808, 2000.”, in which a physical phenomenon caused by a high frequency superposition has been explained through a finite element method, and furthermore, a stable sensor control method at a zero speed and a zero output frequency has been proposed. The magnetic saliency implies a property that an inductance is varied depending on the direction of a magnetic flux or the position of a rotor.
However, such a method also has the following problems. For one of the problems, a high frequency is superposed on a basic voltage. For this reason, the method cannot be used in a place in which a voltage is limited, that is, a control region is limited. For the other problem, the method is a magnetic flux position estimating method based on a high frequency model which is different from and approximates to an electric motor model on the basis of an induction voltage. Therefore, there is a problem in that a torque vibration element such as a voltage error or a noise is generated by the superposition of the high frequency.
Therefore, it is an object of the invention to provide a sensorless control apparatus and method of an AC motor to implement stable torque, speed and position controls within all driving ranges including a region having a zero speed and a zero output frequency without generating a torque vibration element such as a voltage error or a noise even if a high frequency is superposed.